| 摘要 |
For energizing a load (10) with different input voltages, the circuit comprises a transformer (6), whose primary winding (n1), in series with the collector-emitter path of a switching transistor (T1), the load (10), and a current sensing resistor (R2), is arranged between the input-voltage terminals. The seccondary winding (n2), in series with a diode (D1), is arranged in parallel with the load (10). When the input voltage is applied a very small base current is fed to the switching transistor (T1) via the starting resistor (R1), which results in a small current in the primary winding. The resulting voltage rise across the secondary winding (n2) ensures that the switching transistor (T1) is bottomed rapidly via a positive-feedback loop, which comprises the series arrangement of a first resistor (R4), a capacitor (C1) and a second resistor (R5) and which is arranged between that end of the secondary winding (n2) which is connected to the diode (D1) and the base of the switching transistor (T1). For a specific current in the primary winding (n2) the switching transistor (T1) is turned off by means of a control transistor (T2), whose base is connected to the current-sen sing resistor (R2). The energy stored in the transformer (6) is then delivered to the load (10) by an output current in the secondary winding (n2). In order to prevent this output current from increasing strongly at increasing input voltages as a result of the increasing saturation of the switching transistor (T1) via the positive feedback, a zener diode (Z1) is arranged between the junction point of the capacitor (C1) and the resistor (R5) and the base of the switching transistor (T1). Above a specific input voltage this zener diode (Z1) is turned on and drains the base supply current to the emitter of the switching transistor. |
